This blog discusses current weather, weather prediction, climate issues, and other topics

Tuesday, January 20, 2009

Black Ice

What is the the weather condition that kills and injures more Washington State residents than any other? Floods...no. Windstorms..no. Tornadoes or Thunderstorms. No way. I am pretty sure it is roadway icing...and that threat is very real this week. Roadway icing is often called "black ice"--but it really isn't really black...it can look dark at some angles and twinkle in others. Roadway icing usually is associated with high pressure situations, with no rain and light winds. There are several ways you can get ice on the road:

1. Frost--skies clear under high pressure and the earth radiates heat to space via infrared radiation. The surface thus cools sufficiently that frost forms on the ground. Frost can be slippery, but it is generally thin.

2. Fog. The big threat. If the ground cools to below freezing and if some nearby fog passes over the ground, it can rapidly and severely ice up.

During the past few days both of these have occurred. Two days ago, I had frost on the street outside my home. Then the fog bank moved in and a 1/4 of ice covered everything...just treacherous.

Keep in mind that on cold, clear or nearly clear nights the temp of the ground can be 3-6 F cooler than the air temperature at 6 ft, where official measurements are made. So if you hear that temps have dropped to 36F or your car thermometer is down to 35...you should be worried, particularly if fog is around.

Once in a while I have testified at legal proceedings about icing accidents, and many of them have occurred under clear nights, with nearby fog, a bend in the road or a turn, and often someone driving to fast for the conditions.

By the way, the inversion has strengthened over the central Sound and in some places the fog remained most of day (like at the UW). (see graphic of inversion from the profiler). You can easily get out of the murk by going up (above 600 ft should do it). Air quality has declined as result, so no burning if you don't have to.

Finally, a reminder that I will talk about Lowland Snowstorms at Third Place Books in Lake Forest Park tomorrow (Wed) at 7PM.

25 comments:

Anonymous
said...

I got such a laugh when I blew up the picture of your “High Accuracy Thermometer” and saw that it was a Radio Shack indoor/outdoor digital. That was what I used to make the temperature measurement that I sent you from my Tiger hike last week, although I think that my unit is a few years older than yours. I mounted the sensor on the back of my pack to isolate it from my body heat. Also the sensor has low thermal mass so it tracks pretty well when you are on the move. I took care to keep the sun off of the sensor so I felt pretty confident about the numbers that I got.

Hi Josh. I would`nt depend very much on TWC. It`s best to with your local NWS forecast.:o)

Anyway, hard to say what will happen. Short term for late this weekend into early work work week shows mainly dry N-NELY flow and 850mb temps lowering about -9, -10c with surface thicknesses in the upper 510`s(517-518m). So probably some cool/cold days with VERY frosty nights. On the flip side, 0zGFS wants to have a cut-off low south of our region for late next week with a weak ridge over WA/OR, while 12z/00z EURO models show a few cold troughs dipping down over our region for the same time period. So will have to see what really happens.

Brightest, sunniest day yet in Bellingham today. We have yet to see really persistent daytime fog here. Beautiful white frosting last few mornings. Around here, anything can be winter weather. I am no longer surprised by the rare 60 degree December or January day. I also look forward to the reliable warm, spring days in February, April snow storms, 80 degree May days and 50 in June. Interesting always...

@Josh Yes, we had snow, but remember Winter began in the middle of that. So we had about 2 weeks of "winter" then we went back to spring like weather. I seem to remember this last year too with November being more "winter" than winter was, then spring being more "winter" as well.

We have had crystal clear skies in east Woodinville, elev 530' for the past few days, no fog to speak of at all, that's a switch. And you lowlanders are not only in fog, but yellow, nasty, weird fog...

There is confusing information coming from Puget Sound Air Quality. Although news reports are indicating that the air quality has deteriorated to a condition that may affect the health of sensitive persons (orange on the alert status), the metrics on pscleanair’s website do not support this claim. Since Jan. 17, the particulate matter graphs at: http://www.pscleanair.org/airq/aqi.aspx show the air quality never getting into the orange zone. Today, half of the reporting stations are showing the air quality as good. Yet we remain under a stage 2 burn ban. Is the equipment in error? Is there a reporting error? Or is this a case of over-zealous authority? Why is there a stage 2 burn ban with these air quality metrics?

would love to know what next weeks weather could possibly do.....I know the models are all over the place but do any of them hint at any snow that would impede travel or any wild winds, etc? I have to travel for work next week....always seems like the weather acts up when I have to travel!

I've been wondering about that, myself -- regardless of what the Puget Sound Clean Air Agency says about the air quality, I can tell you it's not very good; folks I know with lung diseases like asthma and reactive airway disease are having some difficulty.

The Puget Sound Clean Air Agency is very clear, though, that there's a burn ban in effect -- stage 2, so no indoor burning, either ("unless this is your only adequate source of heat"). We're making do, but won't be able to get it above 60 F inside without the wood stove if the weather gets any colder.

At this point, I'd trade the sunshine for rain if it meant easier breathing and a warmer house!

Begreen's comments convey the complexity of air quality forecasting. Forecasting air quality levels is interesting and counterintuitive. The stage 2 burn ban was issued once measured criteria was met. With no change in the synoptic pattern to break the inversion and restore effective mixing and dispersion expected for >48hrs. Controls are necessary to slow or reverse pollution trends. If you are successful, air quality concentrations should be below the forecast. Forecasts are indicators of expected air quality levels based on predicted meteorology and expected public behavior. For clarity burn bans are only issued on a county wide basis. Fortunately it seems that the public is responding to all of the great coverage of this event and many are choosing to comply with the burn bans. There is a real awareness that our laws permit the lawful use of wood burning devices and PSCAA promptly lifts bans when well mixed weather patterns return. It is noteworthy that the new law permits the issuance of stage 2 bans at levels below the EPA standard. It will not take another decade before the next one is issued.

There is some important physics going on near the surface in black icing conditions. Near surfaces, a film of still air accumulates that acts as an insulation barrier between the general air mass and the surface. It acts very much like the insulating glass cover space of a single glazed solar collector. On a clear night the surface is seeing a cold night sky temperature, determined by the infrared absorption in the water vapor content in the air column. The radiation balance for the surface is the difference between the warm surface radiation and that of the cold air column. Remember that this radiation occurs as the 4th power of the absolute temperature. Insulated by the surface air film, the differential radiation transfer will cool surfaces substantially below air temperatures. I expect that some of the cooler air temperatures measured at the surface is conduction cooling from the surface back into the near surface air. Some, of course, will be density gradient flow as well. This radiation cooling effect is also responsible for fiberglass and metal car porch roofs dripping from the underside in clear nights. The radiation cooling cools the roof surface and condensation from the air forms on both surfaces. The condensate on the bottom surface drips off and is annoying. An insulated or less conducting surface will not experience a temperature difference on the lower surface and will be dryer.

I am wondering the same things as a few other commenters are about the long-er range forecasts. Any wild weather to speak of at all? Wind, lots of rain, more snow, etc. or are we going to stay in this dry period for a while?